On the basis of Śrī Bhagavad-gītā, everything is explained very vividly. The soul is eternal. The soul is transferred from one gross body to another gross body, just like we change our apartment from one apartment to another. But I exist. If I vacate one apartment and I go to another apartment, it does not mean I am finished. I may leave the apartment. Similarly, if we are leaving this body and we are going to another body, that means I am not finished. I am existing. Na hanyate hanyamāne śarīre (BG 2.20). Na hanyate: "The soul is never annihilated, even after the destruction of this body." Therefore the question is that, "If I am eternal, why I am put into this condition of changing, of transmigrating from one body to another? Is there any possibility of not changing the body, to keep eternality?" Yes. That is possible.
Actually we are, as spirit soul, the part and parcel of God. So God is eternal, God is blissful, God is in full knowledge, so we, being part and parcel of God, we have got the same quality.